BackSummary of Reactions of Alkynes (Ch. 9)
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Alkynes as Nucleophiles
Preparation and Reactivity
Alkynes can act as nucleophiles after deprotonation, allowing them to participate in various organic reactions. The terminal alkyne hydrogen is acidic and can be removed to generate a nucleophilic acetylide ion.
Deprotonation: Treat a terminal alkyne with sodium amide (NaNH2) to form an acetylide ion, a strong nucleophile.
Alkylation: The acetylide ion can react with primary alkyl halides in an SN2 reaction to form new carbon-carbon bonds.
Carbonyl Addition: Acetylide ions add to carbonyl compounds:
Formaldehyde yields primary alcohols.
Aldehydes yield secondary alcohols.
Ketones yield tertiary alcohols.
Example: (formation of acetylide ion) (alkylation) (addition to carbonyl)
Formation of Alkynes
Synthesis Methods
Alkynes can be synthesized from alkyl dihalides through elimination reactions.
Terminal Alkynes: Double elimination of vicinal or geminal dihalides using strong base (e.g., NaNH2).
Internal Alkynes: Similar elimination from internal dihalides.
Example: (terminal alkyne formation) (internal alkyne formation)
Additions to Alkynes
Hydrogenation
Alkynes undergo hydrogenation to form alkenes or alkanes, depending on the catalyst and conditions.
Alkyne to Alkane: Complete hydrogenation with Pd/C catalyst.
Alkyne to (Z)-Alkene: Partial hydrogenation using Lindlar's catalyst yields cis-alkenes.
Dissolving Metal Reduction: Sodium in liquid ammonia reduces alkynes to trans-alkenes ((E)-alkenes).
Example: (alkane)
Addition of HX (Hydrohalogenation)
Alkynes react with hydrogen halides (HX) to form vinyl halides and, with excess HX, geminal dihalides.
Without Peroxides: Markovnikov addition, halide adds to the more substituted carbon.
With Peroxides: Anti-Markovnikov addition, halide adds to the less substituted carbon.
Example: (Markovnikov) (Anti-Markovnikov)
Halogenation
Alkynes react with halogens (Br2, Cl2) to form dihaloalkenes and, with excess halogen, tetrahaloalkanes.
One Equivalent: Forms trans-dihaloalkene.
Two Equivalents: Forms tetrahaloalkane.
Example:
Hydration of Alkynes
Alkynes undergo hydration to form carbonyl compounds via tautomerization.
Markovnikov Hydration: Using HgSO4 and H2SO4, forms ketones.
Anti-Markovnikov Hydration: Using BH3/THF followed by H2O2/NaOH, forms aldehydes.
Example: (ketone) (aldehyde)
Summary Table: Major Reactions of Alkynes
Reaction Type | Reagents | Product |
|---|---|---|
Deprotonation | NaNH2 | Acetylide ion |
Alkylation | Alkyl halide | New C–C bond |
Hydrogenation | Pd/C, Lindlar, Na/NH3 | Alkane, cis-alkene, trans-alkene |
Hydrohalogenation | HX (± ROOR) | Vinyl/geminal halide |
Halogenation | Br2, Cl2 | Dihaloalkene, tetrahaloalkane |
Hydration | HgSO4/H2SO4, BH3/H2O2 | Ketone, aldehyde |
